This is a retroactive post. I have already obtained heating element for the hot plate. It comes from a $30 FarberWare hot plate that I got a few years ago when I was into chemistry projects. It has two circular puck elements of different sizes. I took the larger one out of the unit and did some basic measurements for heating time. From 150 C to 220 C it can heat at or above a 1 degree C per second rate, which is pretty ideal for solder reflow during the actual reflow stage. It's also about 8" in diameter which should be sufficient for fairly large PCBs (at least for what I'll be making).

In addition I have already started on the microcontroller code for the project. For this project instead of using basic Arduino libraries I decided to go with register level programming. First time I have done this with AVR chips and am doing it simply for the experience. The requirements for the ucontroller code are as follows:

• Send data over UART port to PC including:
  • Temperature data - which comes in 32-bit packets
• Receive UART data from PC including
  • PWM duty cycle for driving the SSR
  • (optional) Duty cycle for a cooling fan to quickly cool the system down
• 16-bit PWM driver on Timer1 (for ATMEGA238P) for low frequency PWM (~1Hz) to the SSR
• (Optional) PWM channel for the fan
• SPI communication to the MAX31855 to read the temperature of the hot plate from K-type thermocouple
• Safety fallbacks including:
  • Watchdog timer on UART communication - if no new command after a few seconds shut down the system until communication re-established

So far I have the UART communication down and low frequency PWM for the SSR. I am currently working on SPI communication to the MAX31855 which is so far proving difficult. Mostly because I do not have a logic analyzer so I have no clear idea about what bits are actually being sent around on the system.

I will have code posted soon on Github.